In the metal-working industry, processes of forming phosphate coatings by a treatment with aqueous zinc phosphate solutions are used on a large scale. Phosphate coatings formed on metal surfaces which have been treated by such processes serve particularly to reduce sliding friction; to facilitate cold-working; to protect against corrosion; and as a base for paints.
Such phosphate baths usually have a pH value of about 1.8 to about 3.8 and contain mainly zinc and phosphate ions as operative ingredients. In addition to zinc cations, other cations, such as ammonium, calcium, cobalt, iron, potassium, copper, sodium, magnesium, manganese, may be present in such processes. To accelerate the formation of the phosphate layer, oxidizers, such as bromate, chlorate, nitrate, nitrite, organic nitro compounds, perborate, persulfate, hydrogen peroxide, are generally added to the phosphating baths. It is also possible to use an oxygen-containing gas to oxidize iron(II) to iron(III). In order to optimize the formation of the phosphate layer on certain materials, additives consisting, e.g., of fluoride, silicon fluoride, boron fluoride, citrate and tartrate, may be used. The large number of individual ingredients and of their possible combinations permits a large number of different compositions to be used in a phosphating bath.
Such phosphating baths are usually contacted with the workpiece surface to be treated by dipping, flooding or spraying. During the contacting time, which may amount to between a few seconds and half an hour or more, the chemical reaction with the metal surface results in the formation of crystalline phosphate layers which are firmly intergrown with the metal. Because any residual phosphating solution on the surface would disturb further processing, the phosphating treatment is succeeded by a thorough rinsing with water. In order to avoid a detrimental enrichment of the ingredients of the phosphating baths in the rinsing baths, the latter are replenished with fresh water and contaminated rinsing water is withdrawn as an overflow. The contaminated rinsing water contains pollutants and for this reason must be specially treated before it can be delivered to sewer or to a receiving body of water.
As the need for treatment and disposal of spent rinsing water constitutes a disadvantage in the use of phosphating processes, it has been proposed, e.g., in DE-C-23 27 304, to use a zinc phosphating process in which solutions are employed which are so composed that virtually all components can be precipitated by a treatment with Ca(OH).sub.2. This will greatly facilitate the processing of the rinsing water and will afford the advantage that the processed rinsing water is of high quality and can be re-used in the process. A disadvantage resides, however, in that the required precipitability involves a strong restriction as regards the adaptation of the composition of the phosphating bath to the requirements encountered in practice.
It has been suggested in F. Wilhelm (Metalloberflache, 33 (1979), pages 301 to 307) to effect a cascade rinsing after zinc phosphating and to save so much water that the rinsing water can be used to compensate losses from the zinc phosphating zone. However, it is also stated in this reference that such a concept cannot be reduced to practice for reasons of process technology and economy.
It is therefore an object of this invention to provide a process for the formation of phosphate coatings on metals, particularly on steel, galvanized steel, zinc alloy-plated steel, aluminized steel and aluminum by a treatment with zinc phosphate solutions which contain iron(II) and nitrate ions which is waste water-free and which avoids known disadvantages, particularly those mentioned hereinabove. Additional objects and advantages of the invention will be apparent from the following discussion.